ABSTRACT

The large-scale dynamics of the interstellar medium in spiral galaxies is significantly determined by molecular cloud physics. Inelastic cloud-cloud collisions, coupled with gravitational instabilities generated by the gas cooling, give rise to a net flux of angular momentum which is equivalent to a (gravitational) “viscosity”. By using N–body model calculations of a self-gravitating disk we study the efficiency of collisions coupled with gravitational waves, and its importance for large-scale mass accretion in disk galaxies.

THE IDEA

The mass of the interstellar medium of galaxies is essentially included in an ensemble of clouds, whose physics on a small-scale dominates the dynamical behaviour. In a non-axisymmetric potential, the gas clouds are submitted to gravitational torques, which transfer angular momentum outwards, inducing an accumulation of gas in the central regions. Combes and Gèrin (1985) developed a collision model for the interstellar medium, including a mass spectrum of molecular clouds. If on the other hand, differential rotation is sufficient, the gas is submitted to viscous torques, which also accumulates gas in the central regions (Lesch et al. 1990). It is the aim of our investigation to connect these different approaches, by identifying the angular momentum transport in a non-axisymmetric potential with a viscosity. A study of the exchange of angular momentum and energy between clouds will provide an understanding of the nature of the gas “viscosity”.

OVERVIEW

One–dimensional numerical simulations of a large–scale flow of interstellar gas in galactic spiral density waves were produced using the approach described by Roberts (1969), Shu et al. (1972, 1973), Woodward (1975), Marochnik et al. (1983), Lubow et al. (1986), and Berman et al. (1990).

We have employed a two–phase model for the initial state of the interstellar medium (ISM). Thermal processes and self–gravitation of the gas were taken into account. Magnetic fields were not included in this version of the mathematical model. A discussion on the role of the magnetic field and status of the two–phase model of ISM versus Supernova–dominated model can be found in Marochnik and Suchkov (1991).

We have used the Schmidt model of the galaxy and the following parameters of its spiral structure: Ωp = 23, 24, and 13.5 km s-1 kpc-1 (the angular velocity of the spiral pattern), F = 10% (the gravitational field of a spiral arm). The right-hand side of the energy equation is L = n(nΛ – Γ), where nΛ(T) and Γ are the standard cooling and heating functions, respectively (Penston 1970).

Numerical simulations of the interstellar gas flow have been produced for a number of galactocentric distances, R = 4, 5, 6, 8, 10 kpc and two different values of the initial density of the interstellar gas, n0 = 0.05 and 0.5 cm-3. Figure 1 demonstrates one of the examples simulated.

ABSTRACT

We have made new N-body simulations of ringed, barred spiral galaxies that extend and improve upon the previous work of M. P. Schwarz. Using a comprehensive database of multicolor images of a large sample of ringed galaxies, we are able to morphologically “match” galaxies to simulation frames that differ in bar pattern speed and time step. From these matches, we can reliably identify both low and high pattern speed galaxies.

INTRODUCTION

It is now well-established that the rings commonly observed in disk galaxies are probably caused by orbital resonances with a bar or bar-like potential in the disk (see Buta and Crocker 1991, A. J., 102, 1715, hereafter BC; 1993, A. J., 105, 1344, for recent discussions). In a pioneering N-body investigation, Schwarz (1981, Ap. J., 247, 77) simulated the behavior of gas clouds in two dimensional rotating bar potentials, and identified the outer Lindblad resonance (OLR), the inner 4:1 resonance (UHR), and the inner Lindblad resonance (ILR) as the resonances responsible for the features known as outer, inner, and nuclear rings, respectively. To better understand ring formation and evolution, we have carried out new simulations at many more bar pattern speeds (Ωp) than Schwarz with more particles and an improved treatment of cloud collisions. We also take advantage of a recently acquired database of BVI CCD images of 150 ringed galaxies for comparison.

243 Seyfert galaxies of type 1, 2, or 3 are listed in the Catalogue of Quasars and AGNs of Veron and Veron (1989) having mv ≤ 15 and vrad ≤ 20,000 km s-1. Eight of these Seyfert galaxies show two nuclei (IR 0248-11, Mkn 739, Mkn 266, Mkn 463, Mkn 673, Arp 220, NGC 6240, NGC 7593). The separations of the nuclei are 3 – 10 arcsec corresponding to 2 – 6 kpc.

To confirm the double nuclear structure for these disturbed galaxies we determined the internal velocity field to be sure that they are the result of two galaxies in the late stages of merging (Kollatschny et al. 1991).

Making the additional assumption that the Seyfert galaxies Mkn 231 (Kollatschny et al. 1992) and Mkn 273 are galaxies in the final stage of merging, having strong tidal arms but unresolved nuclei, one can estimate that 4 percent of all Seyfert galaxies are in the merging process.

The luminosities of the multiple nuclei Seyfert galaxies are extremely high in comparison to morphologically undisturbed Seyfert galaxies. In Table 1, mean values of the visual and blue luminosities and of the far-infrared and radio (6 cm) luminosities as well as the Ha fluxes are listed for both classes. We have separated Seyfert 1 and Seyfert 2 galaxies.

In all cases the luminosities of double nucleus Seyfert galaxies are higher by a factor of more than two with respect to ‘undisturbed’ Seyfert galaxies — except in the soft X-ray range (ROSAT) and the UV.

ABSTRACT

The nearby elliptical galaxy NGC 4278 (D = 8.2 h-1 Mpc) has long been known to harbor an active LINER, radio nucleus as well as an extended gas disk. Observations of NGC 4278 in the 21 cm HI line using the VLA and recent deep, long-slit optical spectroscopy of the emission line gas are discussed. The atomic gas disk shows regular rotation and extends to over eleven times the half-light radius Re. Noncircular motions in the central regions can be satisfactorily fit by a triaxial model for the galaxy density distribution in which the gas moves on increasingly elliptic orbits towards the nucleus. These elliptic orbits may help feed gas into the active nuclear source in NGC 4278. The high gas rotational velocities in the outer parts give evidence for a massive, dark halo surrounding this active elliptical galaxy.

INTRODUCTION: WHAT CAN WE LEARN FROM GAS KINEMATICS IN ELLIPTICAL GALAXIES?

Contrary to the early definitions of elliptical galaxies as old stellar systems lacking gas and dust, many early-type systems are now routinely detected in all tracers of the cool interstellar medium. About 60% of ellipticals have optical emission lines from ionized gas (Phillips et al. 1986), 45% show thermal dust emission at 60–100 μ (Knapp et al. 1989), 40% show optical absorption by dust patches (Sadler and Gerhard 1985), and about 20% are detected in HI or CO emission (Knapp 1987; Lees et al. 1991).

ABSTRACT

Near–IR photometry within the nuclei of 3 barred spiral galaxies reveals 63–100° isophote twists between the innermost regions and the PA's of the major bars. New N–body simulations indicate that such twists can be best described as the response of the stars to the influence on the potential of the gas within circumnuclear rings.

INTRODUCTION AND OBSERVATIONS

The clearest consequence of barred potentials is manifest in those galaxies undergoing enhanced nuclear star formation, particularly those possessing circumnuclear rings (CNRs). These rings probably form in the region of the inner Lindblad resonances (ILRs), the accumulation of molecular gas acting to enhance star formation. Isophote twists have been noted previously in the centres of barred potentials, both in the stars (Jarvis et al. 1988) and the gas (Devereux et al. 1992), and a link has been suggested between these twists and CNR's.

We have obtained near–IR (1.2–2.2 μm) photometry within the central regions of the barred spirals NGC 1097, NGC 4736 and NGC 5728. Twists of 63°–100° are observed between the innermost isophotes and the known position angles of the bar/lens components, corresponding to the most unambiguous and extreme such detections in any barred spiral later than S0/a. In NGC 1097 and NGC 5728 such twists are immediately interior to the radii of known CNRs. We have considered three possibilities by which such twists could arise.

In weak–intermediate strength bars, there exist two ILRs. The (stable) x2 orbital family dominate between the ILRs and are orthogonal to the main bar.

ABSTRACT

We have studied the bulge formation process by starbursts in young disk galaxies whose disks and halos are gas-rich. If such galaxies tidally encounter another galaxies, large starbursts are easily induced and create galactic superwinds. We study the interaction between the superwind and the halo gas by using a similarity solution and show that a massive, radiativelly-cooled, gaseous shell is formed and becomes gravitationally unstable. In this way, we expect that shells of stars are formed. In order to study further evolution of these shells and their interaction with the disk, we model both the shell and the disk by using an N-body code. Our numerical results show that a large bulge with de Vaucouleurs' density profile is formed from the shell. We also show that the disk is thickened due to the interaction with the shell. The large bulges and thick disks are very similar to these found in S0 galaxies.

INTRODUCTION

Starburst galaxies release huge energy by frequent supernovae. In some starburst galaxies, hot gas and molecular outflows are observed. These outflows are called superwinds (Heckman et al. 1990; Tomisakak and Ikeuchi 1988; Mac Low and McCray 1989). Since some starburst galaxies are interacting galaxies, it was proposed that the starbursts occurs due to gravitational interaction between galaxies (e.g. Noguchi 1988). Such interactions induce bar formation in a galaxy, and the subsequent gas inflow towards the galactic center, as a result of gas-bar interaction and due to the self-gravity in the gas (e.g. Wada and Habe 1990).

ABSTRACT

We present high resolution spectroscopic measurements of the 2.3 μm CO band of the luminous IRAS galaxies NGC 6240 and Arp 220. A convolution analysis yields velocity dispersions σ (FWHM/2.354) of 355 and 150 km s-1 for NGC 6240 and Arp 220, respectively. The velocity dispersion found for NGC 6240 is amongst the highest ever found in a galaxy and it is probably due to violent relaxation associated with the merging of two galaxies. The stellar velocity dispersion of Arp 220 is much smaller than that inferred from the Brα measurement of DePoy et al (1987). Our result implies that there is no dynamical evidence for an AGN in this galaxy. From the dynamical mass derived from our measurements, we infer an infrared mass-to-light ratio M/LK of 0.3 and 0.1 M⊙/L⊙ for NGC 6240 and Arp 220, respectively. For comparison, the ratio typically found in bulges of normal spiral galaxies is 0.6. These observations suggest that the bulk of the 2.2 μim luminosity emitted from the nucleus of NGC 6240 is associated mostly with normal giant stars while the 2.2 μm continuum of Arp 220 seem to have a significant contribution from young red supergiants.

INTRODUCTION

The velocity dispersion is a fundamental quantity for studying the kinematics and stellar populations of galaxies. In normal galaxies, the velocity dispersion is easily obtained by measuring optical emission or absorption lines. Unfortunately, the situation is more complicated for luminous IRAS galaxies.

This contribution considers the consequences of an active nucleus (AGN) inside a galaxy with a nuclear starburst. General arguments suggest that many AGN generate supersonic winds (Smith 1993a, see also Voit et al. 1993) with velocities vw ∼ 0.1 vo.1c, and the interaction of such a wind with a surrounding starburst is considered below. The large number of quasars indicate that some starburst galaxies should contain the remnant black holes of these “dead” AGN. Even if fueled by only a small amount of gas, the resulting AGN wind can have a significant effect on starburst hydrodynamics.

Note that it is unlikely that a black hole (BH) and subsequent AGN could form due to accretion during the lifetime (10τ10 Myr) of the starburst. Since Eddington limited accretion has a timescale of ∼ 500 Myr, a seed black hole would have to accrete at a rate greater than ∼ 50/τ10 times the Eddington rate to grow substantially.

Mass and energy injection by the supernovae and stellar winds of the starburst will form an outflowing wind. A nuclear wind produced by the AGN will evacuate the central region out to the radius where the mass flux injected by the starburst activity is greater than the mass flux in the nuclear wind. At this point the nuclear wind becomes mass loaded, subsonic, and will merge into the developing starburst wind (Smith 1993b). For a uniform starburst the nuclear wind (with kinetic luminosity will become mass loaded at a radius is the starburst mass injection rate, parameterized as the supernova rate times the mass injected per supernova (including stellar winds).

ABSTRACT

Using new high-resolution imaging at optical wavelengths, we study the morphology of the inner kpc of two barred spirals, NGC 4321 and NGC 4314. In the case of NGC 4321, we present evidence for the existence of a nested bar and a nested spiral structure. In the case of NGC 4314, the inner star formation activity is not organized in spiral arms, but has a ring-like appearance. The role of large scale bars and of possible nested bar structure in the fueling of nuclear starbursts is briefly discussed.

INTRODUCTION

The need to dissipate angular momentum of material flowing into the central region of a galaxy is one of the problems in the process of fueling active galactic nuclei (AGN). Non-axisymmetric potentials are capable of achieving an efficient dissipation of angular momentum, and bars may well trigger the nuclear activity of galaxies (see e.g. Simkin et al. 1980). Shlosman et al. (1989) proposed a mechanism where a set of nested bars can play an important role in this process. “Bars within bars”, possibly of a similar kind to those proposed by Shlosman et al., have been observed in a number of galaxies (see e.g. de Vaucouleurs 1974; Friedli and Pfenniger 1993 and references therein).

We show here preliminary results from an imaging study of two barred spiral galaxies which show a mild form of nuclear activity: a (circum-)nuclear starburst. These galaxies, NGC 4314 and NGC 4321, are well-known cases where strong formation of massive stars takes place in a region within about 1 kpc in radius from the center (see e.g. Pogge 1989).

ABSTRACT

Observational evidence for the existence of bars in the central parts of Seyfert galaxies Mkn 744, Mkn 573, and NGC 4151 is presented.

INTRODUCTION

Afanasiev (1981) investigated rotation curves of 28 Seyfert galaxies and found local maxima at distances 0.5–2 kpc from the centers in 20 objects. The rotation velocity declined with the increasing distance from the local maximum more rapidly than for the Keplerian law. This suggests a possibility of development of Kelvin-Helmholtz instability leading to essential turbulization of the circumnuclear disk (global gas instability in the disk) (Morozov 1979). Observed solid body rotation, from the center to the local maximum, indicates a possible presence of bars in the centers of Sy s.

GAS MOTION FEATURES IN THE BAR'S REGION

Gaseous disk in the region of the bar is thinner than the latter, has lower chaotic velocities and rotates faster than the bar Ωd > Ωb (Fridman 1987). If the radius R1, where the relative linear velocity ΔVR reaches the sound speed in the gaseous disk ΔVR = (Ωd – Ωb)R > Vs, is within the bar region (Rb), then a shock may arise for R1 < Rb. The shock front then will be observed on the rear side of the bar with respect to the direction of rotation, if the disk and the bar rotate in the same direction. Depending on the ΔVR value, either strong or weak shocks will be observed in the optical band (mainly in the forbidden lines) as linear structures.

ABSTRACT

Recent observations of non-circular cold gas motions in the central 100 pc — 1 kpc regions of galaxies are summarized, with an emphasis on relating the observed local motions to net inflow and outflow rates.

INTRODUCTION

Non-circular gas motions have been detected in many galaxies, providing empirical evidence that radial gas flows continue to affect the evolution of galaxies long after the initial epoch of galaxy formation. This paper reviews observations of non-circular motions of the cold (molecular and atomic) gas in the 100 pc – 1 kpc circumnuclear regions of disk galaxies. This is typically the region of galaxies where bulges begin to dominate the potential, where rotation curves change from steeply rising to flat, and where circumnuclear starbursts occur. Molecular gas is generally the dominant phase of the interstellar medium by mass in the central regions of gas-rich disk galaxies, and for this reason many of the observations discussed in this review are of CO or other tracers of molecular gas. The weakness of HI emission in circumnuclear regions, and the related difficulty of determining HI kinematics at high resolution, make HI results less common but valuable. Although on average inflowing gas is probably colder than outflowing gas, there are examples of both outflowing molecular gas and inflowing ionized gas.

The relationships between the observed local non-circular motions and the net radial flow rates vary considerably, and in several important cases the net flow rates are much smaller than the local non-circular motions.

ABSTRACT

VIA H I observations of the interacting pair IC 2163/NGC 2207 are presented. The velocity and structural anomalies of IC 2163 agree with predictions of N-body galaxy encounter simulations if IC 2163 recently underwent a strong, prograde, in-plane encounter with NGC 2207. The velocity disturbances in NGC 2207 suggest that the main tidal force on NGC 2207 was perpendicular to the disk of NGC 2207.

INTRODUCTION

The spiral galaxies IC 2163 and NGC 2207, shown in Figure 1, are involved in a close tidal encounter. IC 2163 has an ocular shape (an eye-shaped central oval with a sharp apex at each end), intense star formation along the eyelid regions, and a double-parallel arm structure on the side opposite its companion, NGC 2207.

The optical morphology of IC 2163 is consistent with N-body simulations by Elmegreen et al (1991) if IC 2163 underwent a strong, in-plane, prograde encounter with NGC 2207 during the last half-rotation. One component of the double arm is the usual tidal tail; the other component is produced by rapid streaming of tidally perturbed stars and gas from the companion (western) side of the galaxy. The simulations predict a velocity difference of 50 to 100 kms-1 between the two components of the double arm and large streaming motions along the oval.

Using the VLA, we made H I observations of this galaxy pair to study the early stages of post-encounter evolution and to look for the velocity anomalies predicted by the numerical simulations.

ABSTRACT

We are studying the optical, radio, and X-ray morphologies of interacting galaxies in which at least one member is also an active galaxy. Deformed gas distributions are seen in galaxies that also show optical evidence of tidal deformation, indicating significant gas redistribution in these interacting systems, thereby providing compelling evidence for a causal connection between tidal and nuclear activity in “interactive” galaxies.

THE INTERACTION-ACTIVITY CONNECTION

Stockton (1990) and Heckman (1990) have reviewed the wealth of evidence indicating that galaxy interactions are somehow related to the generation of starburst and AGN activity in galactic nuclei. For example, a large fraction of low-luminosity radio and active galaxies have nearby companions or show evidence for a recent gravitational encounter (e.g., MacKenty 1989). In an optical study of galaxies selected on the basis that they all contain well defined radio jets, it was found that almost half of the sample consists of pairs of elliptical galaxies (Colina and Pérez-Fournon 1990a, b). Many of these low-luminosity radio galaxies with companions (e.g., 3C31, 3C278, and 3C449) show a well defined distorted radio jet structure at the VLA scale with an S– or C-shaped morphology. We are currently studying these systems across many wavebands (optical, radio, and X-ray) in an attempt to model the observed morphologies in each case and thereby constrain the various properties of the system (orbital parameters, jet parameters, and hot gas distribution, respectively). These system constraints will hopefully offer some physical insight into the interaction-activity connection.

Broad-Line Radio Galaxies (BLRG) are the most extreme species of AGN regarding line width and structure of their optical emission line spectra. The FWZI amounts to Δv ≃ 35,000 km s-1 for some objects. Furthermore, the broad line profiles are more structured than these of Seyfert galaxies and show often double hump features (Chen, Halpern, and Filippenko 1989; Halpern 1990; Veilleux and Zheng 1991). The detailed analysis of the line profile variations of these AGN provides a powerful tool to get information about fundamental parameters of the Broad-Line Region (BLR) like size, geometry and especially the kinematics of the line emitting gas (Robinson, Pérez, and Binette 1990; Welsh and Home 1991).

In 1989 we started a monitoring campaign of line profile variations of Broad-Line Radio Galaxies at Calar Alto Observatory/Spain on time scales of weeks to years. In the following we present some examples of emission line profile variations of BLRGs we are studying.

3C390.3 is a well known BLRG with prominent double-peaked Hα emission line profile. The spectra shown in Figure 1 were taken in October 89 and August 92. The line flux was scaled with respect to the narrow forbidden [OIII]λλ4959, 5007 and [OI]λ6300 emission lines. The corresponding difference spectrum is displayed at the bottom of Figure 1. The Hα and the Hβ line profiles show a double-peak structure in the difference spectrum. The humps are separated by Δv ≃ 9,000 km s-1. A component at vrel = 4600 km s-1 described by Veilleux and Zheng (1991) is not visible for these epochs.